Non-murine cells have been productively infected with various high oncogenic mouse mammary tumor viruses (MMTVs) and are being used to distinguish viral-coded from host-derived determinants. Radioimmunoassays (RIAs) for the major external and internal proteins of MMTVs demonstrated that both contain type specific as well as group specific reactivities. Polypeptide map analyses of MMTV proteins substantiate these findings. A new type-B retrovirus has been isolated from Mus cervicolor that shares some antigenic reactivity with known MMTVs. RIAs have also been developed for type-D retroviruses of three primate species. Broad reactivity type-D and type-B interspecies assays are being employed to characterize new antigenic reactivities in both murine and primate tissues. The establishment of these studies are: (a) to analyze the antigenic reactivities of the structural proteins of type-B and type-D retroviruses; (b) to develop interspecies radioimmunoassays which detect cross-reacting antigenic determinants in a broad range of species; (c) to use these radioimmunoassays to investigate the possible involvement of viruses in the etiology of carcinomas in experimental animals and humans.